Vehicles commonly employ variable ratio transmissions to transfer power between an internal combustion engine and the vehicle wheels. In an automatic transmission, a controller selects the transmission ratio in response to the vehicle speed and a driver demand, usually communicated by depressing an accelerator pedal. In certain architectures, the vehicle also has a traction motor connected at the input of the transmission. The traction motor is electrically connected to a traction battery, typically a high-voltage battery having a terminal voltage greater than 100V. The motor can be used in either a motoring mode in which energy from the battery is used to supplement the engine power or in a generating mode in which the motor converts mechanical energy into electrical energy which is stored in the battery. Some vehicles also include a starter/generator, such as a belt integrated starter/generator (BISG) electrically connected to an auxiliary battery, typically a low-voltage battery having a terminal voltage less than 100V (e.g., 12V or 48V). A vehicle with a BISG may supply a charge to the low-voltage battery by either the BISG or the traction motor/traction battery.